Systems and methods for controlling power in an electrosurgical generator

ABSTRACT

The electrosurgical systems and corresponding methods of the present disclosure involve an electrosurgical generator, sensing circuitry, and a controller. The electrosurgical generator includes a radio frequency (RF) output stage that supplies power to tissue. The sensing circuitry measures impedance of tissue. The controller controls the power supplied from the RF output stage to track a nonlinear power curve until the power supplied from the RF output stage has reached a predetermined peak power of the nonlinear power curve. The controller further determines whether a tissue reaction has occurred based on impedance measured by the sensing circuitry and controls the power supplied from the RF output stage during a cooling phase if the controller determines that a tissue reaction has occurred. The controller may further control the power supplied from the RF output stage to track a linear power curve.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of and priority to U.S.Provisional Application Ser. No. 62/151,655, filed on Apr. 23, 2016, theentire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to electrosurgical systems and methodsfor performing electrosurgery. More particularly, the present disclosurerelates to controlling the delivery of power to tissue during the cookstage of a tissue sealing procedure.

2. Description of Related Art

Electrosurgery involves application of high radio frequency (RF)electrical current to a surgical site to seal, cut, ablate, or coagulatetissue. In monopolar electrosurgery, a source or active electrodedelivers RF energy from the electrosurgical generator to the tissue anda return electrode (e.g., a return pad) carries the current back to thegenerator. In monopolar electrosurgery, the active electrode istypically part of the surgical instrument held by the surgeon andapplied to the tissue to be treated. The patient return electrode isplaced remotely from the active electrode to carry the current back tothe generator.

In bipolar electrosurgery, one of the electrodes of the hand-heldinstrument functions as the active electrode and the other as the returnelectrode. The return electrode is placed in close proximity to theactive electrode such that an electrical circuit is formed between thetwo electrodes (e.g., electrosurgical forceps). In this manner, theapplied electrical current is limited to the body tissue positionedbetween the electrodes. When the electrodes are sufficiently separatedfrom one another, the electrical circuit is open and thus inadvertentcontact of body tissue with either of the separated electrodes does notcause current to flow.

Bipolar electrosurgery generally involves the use of forceps. A forcepsis a pliers-like instrument which relies on mechanical action betweenits jaws to grasp, clamp and constrict vessels or tissue. So-called“open forceps” are commonly used in open surgical procedures whereas“endoscopic forceps” or “laparoscopic forceps” are, as the name implies,used for less invasive endoscopic surgical procedures. Electrosurgicalforceps (open or endoscopic) utilize mechanical clamping action andelectrical energy to affect hemostasis on the clamped tissue. Theforceps include electrosurgical conductive plates which applyelectrosurgical energy to the clamped tissue. By controlling theintensity, frequency and duration of the electrosurgical energy appliedthrough the conductive plates to the tissue, the surgeon can coagulate,cauterize and/or seal tissue.

Tissue or vessel sealing is a process of liquefying the collagen,elastin, and ground substances in the tissue so that they reform into afused mass with significantly-reduced demarcation between the opposingtissue structures. Cauterization involves the use of heat to destroytissue and coagulation is a process of desiccating tissue wherein thetissue cells are ruptured and dried.

Tissue sealing procedures involve more than simply cauterizing orcoagulating tissue to create an effective seal; the procedures involveprecise control of a variety of factors. For example, in order to affecta proper seal in vessels or tissue, two predominant mechanicalparameters must be accurately controlled: the pressure applied to thetissue and the gap distance between the electrodes (i.e., the distancebetween opposing jaw members or opposing sealing plates). In addition,electrosurgical energy must be applied to the tissue under controlledconditions to ensure the creation of an effective vessel seal.Techniques have been developed to control or vary the power of energyapplied to the tissue during the tissue sealing process. When a targettissue impedance threshold is reached, the tissue seal is deemedcompleted and the delivery of electrosurgical energy is halted.

The power control systems of conventional electrosurgical generatorsinclude nonlinearities that may impact the quality and consistency oftissue seals. To overcome these nonlinearities, electrosurgicalgenerators have been designed to include high performance controlsystems. These high performance control systems use field-programmablegate array (FPGAs) technology to allow for increased generator controlspeeds so that the high performance control systems can overcome thenonlinearities of conventional power control systems.

SUMMARY

The present disclosure features an electrosurgical generator,comprising, a radio frequency (RF) output stage that supplies power totissue; sensing circuitry that measures impedance of tissue; and acontroller which controls the power supplied from the RF output stage totrack a nonlinear power curve until the power supplied from the RFoutput stage has reached a predetermined peak power of the nonlinearpower curve; determine whether a tissue reaction has occurred based onimpedance measured by the sensing circuitry; and control the powersupplied from the RF output stage during a cooling phase if thecontroller determines that a tissue reaction has occurred.

In one aspect, the controller further controls the power supplied fromthe RF output stage to track a linear power curve if the controllerdetermines that the power supplied from the RF output stage has reachedthe predetermined peak power of the nonlinear power curve. In anotheraspect, the controller includes a memory storing a look-up tableincluding a plurality of nonlinear power curves, a respective pluralityof linear power curves, and a respective plurality of sizes ofelectrodes of electrosurgical instruments usable with theelectrosurgical generator, and the controller receives an electrode sizeand selects a nonlinear power curve from the plurality of nonlinearpower curves and a respective linear power curve from the plurality oflinear power curves based on the received electrode size.

In other aspects, the controller further adjusts a parameter of thelinear power curve based on a surface area of an electrode of anelectrosurgical instrument usable with the electrosurgical generator. Inother aspects, the parameter of the linear power curve is selected fromthe group consisting of slope and duration. In other aspects, theduration of the linear power curve is zero. In other aspects, the linearpower curve has a shorter duration and a larger slope for a largersurface area of the electrode. In other aspects, the linear power curvehas a longer duration and a smaller slope for a smaller surface area ofthe electrode.

In other aspects, the controller further adjusts a parameter of thenonlinear power curve based on a surface area of an electrode of anelectrosurgical instrument usable with the electrosurgical generator. Inother aspects, the parameter of the nonlinear power curve is selectedfrom the group consisting of starting power, duration, shape, slopes,the predetermined peak power, and combinations thereof. In otheraspects, the nonlinear power curve has a longer duration and a smallerpredetermined peak power for a larger surface area of the electrode. Inother aspects, the nonlinear power curve has a shorter duration and alarger predetermined peak power for a smaller surface area of theelectrode. In further aspects, the electrosurgical instrument includes aRadio Frequency Identification tag storing the parameter of thenonlinear power curve. In another aspect, the nonlinear power curve is athird-order or cubic polynomial defined by a plurality of coefficients.

In other aspects, the controller determines a minimum impedance based onthe measured impedance. In other aspects, the controller furtherdetermines whether a tissue reaction has occurred based on the minimumimpedance and a predetermined rise in impedance of tissue being treated.In other aspects, the controller further determines whether a tissuereaction has occurred within a first predetermined period, stop thepower supplied from the RF output stage, and issue a re-grasp message,if the controller determines that a tissue reaction has occurred withinthe first predetermined period. In other aspects, the controller furtherdetermines whether a tissue reaction has occurred within a secondpredetermined period, and control the power supplied from the RF outputstage to restart tracking of the nonlinear power curve if the controllerdetermines that a tissue reaction has not occurred within the secondpredetermined period. Any one or more of the above aspects of thepresent disclosure may be combined without departing from the scope ofthe present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects and features of the presently disclosed system and method willbecome apparent to those of ordinary skill in the art when descriptionsof various embodiments thereof are read with reference to theaccompanying drawings, wherein:

FIG. 1A is a perspective view of an electrosurgical system including anelectrosurgical generator and an electrosurgical instrument having anend effector, according to embodiments of the present disclosure;

FIG. 1B is a rear, perspective view of the end effector of FIG. 1A shownwith tissue disposed between the jaw members of the end effector;

FIG. 2 is a schematic block diagram of a generator control systemaccording to the present disclosure;

FIG. 3A is a flow chart showing a first portion of a tissue sealingmethod according to embodiments of the present disclosure;

FIG. 3B is a flow chart showing a second portion of a tissue sealingmethod according to embodiments of the present disclosure; and

FIG. 4 is a graph illustrating changes in supplied energy during asealing procedure utilizing the method shown in FIGS. 3A and 3B.

DETAILED DESCRIPTION

The electrosurgical systems and methods of the present disclosurecontrol the power of electrosurgical energy output from anelectrosurgical generator so that the power follows or tracks powercontrol curves having a non-linear power control curve and a linearpower control curve during a “cook stage” of an electrosurgicalprocedure. These power control curves stem from the observation thattissue volume and the electrosurgical energy required to heat thattissue volume have a non-linear relationship. According to embodimentsof the present disclosure, the non-linear power control curve, such as apolynomial power control curve, is tuned to provide an initially lowrate of increase of power to the tissue, but later compensates for thelow rate of increase of power by a higher rate of increase of power.

FIG. 1A is a schematic illustration of an electrosurgical system 1.Electrosurgical system 1 includes forceps 10 for treating patienttissue. Electrosurgical energy is supplied to forceps 10 by anelectrosurgical generator 2 via a cable 18. This allows the user toselectively coagulate and/or seal tissue.

As shown in FIG. 1A, forceps 10 is an endoscopic version of a vesselsealing bipolar forceps. Forceps 10 is configured to support an endeffector assembly 100 and generally includes a housing 20, a handleassembly 30, a rotating assembly 80, and a trigger assembly 70 thatmutually cooperate with the end effector assembly 100 to grasp, seal,and if required, divide tissue. Forceps 10 also includes a shaft 12 thathas a distal end 14, which mechanically engages the end effectorassembly 100, and a proximal end 16 which mechanically engages thehousing 20 proximate to the rotating assembly 80.

Forceps 10 also includes a plug (not shown) that connects forceps 10 toa source of electrosurgical energy, e.g., electrosurgical generator 2,via cable 18. Handle assembly 30 includes a fixed handle 50 and amovable handle 40. Handle 40 moves relative to fixed handle 50 toactuate the end effector assembly 100 and enable a user to selectivelygrasp and manipulate tissue 400, as shown in FIG. 1B. Forceps 10 mayalso include an identification module (not explicitly shown) such as aresistor or computer memory or a Radio-frequency identification (RFID)or barcode readable by the generator 2 to identify the forceps.

Referring to FIGS. 1A and 1B, end effector assembly 100 includes a pairof opposing jaw members 110 and 120. Electrically conductive sealingplates 112 and 122 are attached to jaw members 110 and 120,respectively. Electrically conductive sealing plates 112 and 122 conductelectrosurgical energy through tissue 400 held between jaw members 110and 120. Jaw members 110 and 120 move in response to movement of handle40 from an open position to a closed position. In the open position,electrically conductive sealing plates 112 and 122 are disposed inspaced relation relative to one another. In the closed or clampingposition, electrically conductive sealing plates 112 and 122 cooperateto grasp tissue and apply electrosurgical energy to the grasped tissue.In embodiments, end effector assembly 100 includes a jaw angle sensor(not shown) adapted to sense the angle 114 between jaw members 110 and120 and configured to operably couple to electrosurgical generator 2.

Jaw members 110 and 120 are activated using a drive assembly (not shown)enclosed within the housing 20. The drive assembly cooperates with themovable handle 40 to move jaw members 110 and 120 from the open positionto the closed position. Examples of handle assemblies are shown anddescribed in commonly-owned U.S. Pat. No. 7,156,846 entitled “VESSELSEALER AND DIVIDER FOR USE WITH SMALL TROCARS AND CANNULAS,” thecontents of which are hereby incorporated by reference herein in theirentirety.

The handle assembly 30 may include a four-bar mechanical linkage whichprovides a unique mechanical advantage when sealing tissue between jawmembers 110 and 120. For example, once the desired position for thesealing site is determined and jaw members 110 and 120 are properlypositioned, handle 40 may be compressed fully to lock electricallyconductive sealing plates 112 and 122 in a closed position against thetissue. The details relating to the inter-cooperative relationships ofthe inner-working components of forceps 10 are disclosed in theabove-cited commonly-owned U.S. Pat. No. 7,156,846, which discloses anoff-axis, lever-like handle assembly.

Forceps 10 also includes a rotating assembly 80 mechanically associatedwith the shaft 12 and the drive assembly (not shown). Movement of therotating assembly 80 imparts similar rotational movement to the shaft 12which, in turn, rotates the end effector assembly 100. Various featuresalong with various electrical configurations for the transference ofelectrosurgical energy through the handle assembly 30 and the rotatingassembly 80 are described in more detail in the above-mentionedcommonly-owned U.S. Pat. No. 7,156,846.

As shown in FIGS. 1A and 1B, end effector assembly 100 attaches to thedistal end 14 of shaft 12. Jaw members 110 and 120 are pivotable about apivot 160 from the open to closed positions upon relative reciprocation,i.e., longitudinal movement, of the drive assembly (not shown). Again,examples of mechanical and cooperative relationships with respect to thevarious moving elements of the end effector assembly 100 are furtherdescribed in the above-mentioned commonly-owned U.S. Pat. No. 7,156,846.

Forceps 10 may be designed such that it is fully or partially disposabledepending upon a particular purpose or to achieve a particular result.For example, end effector assembly 100 may be selectively and releasablyengageable with the distal end 14 of the shaft 12 and/or the proximalend 16 of the shaft 12 may be selectively and releasably engageable withthe housing 20 and handle assembly 30. In either of these two instances,forceps 10 may be either partially disposable or replaceable, such aswhere a new or different end effector assembly 100 or end effectorassembly 100 and shaft 12 are used to selectively replace the old endeffector assembly 100 as needed.

The electrosurgical generator 2 includes input controls (e.g., buttons,activators, switches, touch screen, etc.) for controlling the generator.In addition, the electrosurgical generator 2 includes one or moredisplay screens for providing the surgeon with a variety of outputinformation (e.g., intensity settings, treatment complete indicators,etc.). The controls allow the surgeon to adjust parameters of the RFenergy output by the electrosurgical generator 2, including the powerand the waveform, to achieve a desired result suitable for a particulartask (e.g., coagulating, tissue sealing, division with hemostatis,etc.). Forceps 10 may include a plurality of input controls which may beredundant with certain input controls of the electrosurgical generator2. Placing the input controls at forceps 10 allows for easier and fastermodification of RF energy parameters during the surgical procedurewithout requiring interaction with the electrosurgical generator 2.

FIG. 2 shows a schematic block diagram of the electrosurgical generator2 having a controller 4, a high voltage DC power supply 7 (“HVPS”), anRF output stage 8, and sensor circuitry 11. HVPS 7 provides DC power toan RF output stage 8 which then converts DC power into RF energy anddelivers the RF energy to forceps 10. Controller 4 includes amicroprocessor 5 operably connected to a memory 6 which may be volatiletype memory (e.g., RAM) and/or non-volatile type memory (e.g., flashmedia, disk media, etc.). Microprocessor 5 includes an output port whichis operably connected to the HVPS 7 and/or RF output stage 8 allowingthe microprocessor 5 to control the output of the generator 2 accordingto either open and/or closed control loop schemes. A closed loop controlscheme may be a feedback control loop wherein the sensor circuitry 11provides feedback to controller 4 (i.e., information obtained from oneor more of sensing mechanisms for sensing various tissue parameters suchas tissue impedance, tissue temperature, fluid presence, output currentand/or voltage, etc.). Controller 4 then signals the HVPS 7 and/or RFoutput stage 8 which then adjusts DC power supply and/or RF powersupply, respectively. The controller 4 also receives input signals fromthe input controls of the generator 2 and/or forceps 10. Controller 4utilizes the input signals to adjust the power output of the generator 2and/or instructs generator 2 to perform other control functions.

It is known that sealing of the tissue 400 is accomplished by virtue ofa unique combination of gap control, pressure, and electrical control.In particular, controlling the intensity, frequency, and duration of theelectrosurgical energy applied to the tissue through the electricallyconductive sealing plates 112 and 122 are important electricalconsiderations for sealing tissue. In addition, two mechanical factorsplay an important role in determining the resulting thickness of thesealed tissue and the effectiveness of the seal, i.e., the pressureapplied between jaw members 110 and 120 (between about 3 kg/cm2 to about16 kg/cm2) and the gap distance “G” between electrically conductivesealing plates 112 and 122 of jaw members 110 and 120, respectively,during the sealing process (between about 0.001 inches to about 0.006inches). One or more stop members 90 may be employed on one or bothsealing plates to control the gap distance. A third mechanical factorthat contributes to the quality and consistency of a tissue seal is theclosure rate of the electrically conductive surfaces or sealing platesduring electrical activation.

Since forceps 10 applies energy through electrodes, each of jaw members110 and 120 includes a pair of electrically conductive sealing plates112, 122, respectively, disposed on an inner-facing surface thereof.Thus, once jaw members 110 and 120 are fully compressed about tissue400, forceps 10 is ready for selective application of electrosurgicalenergy as shown in FIG. 4. Upon the application of electrosurgicalenergy, electrically conductive sealing plates 112 and 122 cooperate toseal tissue 400 held between electrically conductive sealing plates 112and 122.

Electrosurgical system 1 according to the present disclosure regulatesapplication of energy and pressure to achieve an effective seal capableof withstanding high burst pressures. Electrosurgical generator 2applies energy to tissue during the cook stage based on a power controlcurve or a current control curve. Two examples of power control curvesare illustrated in FIG. 4. Power control curve 404 may be used forsmaller-sized jaw members while power control curve 406 may be used forlarger-sized jaw members. As described below, the point of transitionfrom the polynomial portion of the power control curve to the linearportion of the power control curve (e.g., at the peak of the polynomialportion) and/or other parameters of the power control curve may beadjusted as a function of the size of jaw members 110 and 120.

A RFID or barcode tag may be attached to one of jaw members 110 and 120to provide the parameters for the power control curve, For example,through the use of the RFID or barcode tags, electrical generator 2 mayuse the specific information found in the RFID or barcode tag to selectfrom a lookup table the parameters for the power control curve prior toapplication of the power. The parameters used in implementation of thepower control curve which controller 4 tracks include but are notlimited to: the peak of the polynomial portion of the power controlcurve or the point of transition from the polynomial portion of thepower control curve to the linear portion of the power control curve,the minimum starting power of the power control, the coefficients (e.g.,a, b, c, and d used in equation (1) below), minimum time during thepolynomial portion of the power curve or minimum time during the cookstage, slope of the linear portion of the power control curve,

The control system of the present disclosure includes controller 4 andmemory 6. This control system regulates the electrosurgical energyoutput by the RF output stage 8. For example, the control system mayvary characteristics of the electrosurgical energy (e.g., power, voltagebeing maintained, duration of power application, etc.) based on the typeof tissue being sealed. The control system may vary the duration of theapplication of power depending on the size, thickness, or othercharacteristics of the tissue. For example, the control system may use alonger duration of the application of power for thicker tissue or ashorter duration of the application of power for thinner tissue. Inembodiments, the control system may adjust the power of theelectrosurgical energy based on the jaw angle.

As mentioned above, various methods and devices are contemplated toautomatically regulate the closure of jaw members 110 and 120 abouttissue to keep the pressure constant during the sealing process. Forexample, forceps 10 may be configured to include a ratchet mechanism(not explicitly shown) which initially locks jaw members 110 and 120against the tissue under a desired tissue pressure and then increasesthe pressure according to the command from the microprocessor 5 to anoptimum tissue pressure. The ratchet mechanism (not explicitly shown) isconfigured to adjust the pressure based on electrical activation and/orthe tissue reaction. The pressure may be controlled in a similar mannertowards the end of the seal cycle, i.e., release pressure. The pressuremay be held constant or varied during a cooling period. A similar or thesame ratchet mechanism (not explicitly shown) may be employed for thispurpose as well. The ratchet mechanism (not explicitly shown) may beconfigured to automatically release or unlock at the end of a coolingperiod. Other controllable closure mechanisms or pressure-applyingmechanism are also envisioned which may be associated with the handleassembly 30, the housing 20 and/or jaw members 110 and 120. Any of thesemechanisms may be housed in the housing 20 or form a part of eachparticular structure. The ratchet, closure, and/or pressure-applyingmechanism may include any suitable actuating device, for example withoutlimitation, a solenoid, stepper motor, vacuum actuator, and/or apressure actuator.

From the foregoing and with reference to the various drawings, thoseskilled in the art will appreciate that certain modifications can alsobe made to the present disclosure without departing from the scope ofthe same. Further details relating to one particular open forceps aredisclosed in commonly-owned U.S. Pat. No. 7,811,283 entitled “OPENVESSEL SEALING INSTRUMENT WITH CUTTING MECHANISM AND DISTAL LOCKOUT,”the entire contents of which are incorporated by reference herein.

FIG. 3A is a flowchart illustrating a first portion of a tissue sealingprocess according to embodiments of the present disclosure. In step 300,the tissue sealing procedure is activated (e.g., by pressing of a footpedal or hand-switch) and a host processor (e.g., microprocessor 5)activates a tissue sealing control system and loads a configurationfile, e.g., from the generator's internal memory or from an RFID tagattached to the instrument or forceps. The configuration file includessettings for the control system, e.g., an end impedance threshold (e.g.,EndZ), baseline cooling time (e.g., Base_Cool_T), a forceps/instrumentidentification (e.g., ForcepsID), and coefficients defining the powercurve for the cook stage. At least some of the settings may depend onthe tissue type. In embodiments, the configuration file may includesettings corresponding to different tissue types. In those embodiments,when the control system determines the tissue type, e.g., through userinput or by using a detector that detects tissue type, the controlsystem selects settings for the determined tissue type. In embodiments,some of the settings of the configuration file may be adjusted based onthe instrument being used or the settings selected by a surgeon.

The configuration file may be loaded from a data store included withincontroller 4. Additionally or alternatively, the configuration file maybe loaded from a data store, e.g., a nonvolatile memory or RFID tag,included within forceps 10. In embodiments, a plurality of configurationfiles may be included within controller 4. A configuration file may beselected and loaded in accordance with the type of forceps beingutilized, e.g., as determined based on the ForcepsID. In embodiments,forceps 10 are interrogated by controller 4 to ascertain the ForcepsID,whereupon a configuration file corresponding to the ForcepsID is loaded.The baseline cooling time may be determined in accordance withForcepsID. In step 300, the predetermined impedance rise value (e.g.,Z_Rise) is loaded from the configuration file. The predeterminedimpedance rise value (Z_Rise) value may range from about 1 ohm to about750 ohms and its specific value is based on the electrosurgicalgenerator 2 used and tissue being sealed. Additionally, in step 300, thecontrol system records the lowest impedance value (e.g., Z_Low) based onthe measured impedance and stores the lowest impedance value (Z_Low) inmemory.

In step 302, the process begins with an impedance sense phase. In step302, the process measures the tissue impedance using an interrogatoryimpedance sensing pulse of a predetermined duration, e.g., approximately100 ms. The value of the median measured tissue impedance is stored as astart impedance value (e.g., Start_Z). Tissue impedance is determinedwithout appreciably changing the tissue. During this interrogation orerror-checking phase, the electrosurgical generator 2 checks for an opencircuit in step 304, to determine whether or not tissue is beinggrasped.

The cumulative amount of energy delivered to the tissue during thesealing procedure may be stored as a total energy delivery variable(e.g., E_Total). E_Total may be determined in any suitable manner, forexample, by integrating the output power over a power delivery time. Inembodiments, the output power is sampled and totaled on a periodic basisto obtain an approximation of the total energy delivery. Microprocessor5 may be configured to execute an interrupt service routine (ISR) thatis programmed to periodically measure and sum or total the cumulativeoutput power (E_Total). In embodiments, maximum energy delivery rate(e.g., E_Max), minimum energy delivery rate (e.g., E_Min), and averageenergy delivery rate (e.g., E_Avg) may also be computed and stored.

Thermal properties related to the tissue may be measured, recorded,and/or computed during the sealing process. Such properties may include,without limitation, total thermal energy sensed, which may be expressedas the sensed temperature integrated over the time of the procedure(e.g., T_total), maximum tissue temperature (e.g., T_Max), minimumtissue temperature (e.g., T_Min), and average tissue temperature (e.g.,T_Avg). Fluid properties, i.e., a total quantity of fluid, which may beexpressed as the sensed quantity of fluid integrated over the time ofthe procedure (e.g., F_Total), a maximum fluid quantity (e.g., F_Max), aminimum fluid quantity (e.g., F_Min), and an average fluid quantity offluid (e.g., F_Avg), may additionally or alternatively be sensed,recorded and/or computed.

To determine whether there is an open circuit in step 304, the processdetermines whether the measured impedance is greater than a highimpedance threshold or less than a low impedance threshold. If themeasured impedance is greater than the high impedance threshold, an opencircuit is detected.

Alternatively or additionally, the phase between voltage and current ismeasured by measuring the phase of the voltage with respect to thecurrent or by measuring the phase of the current with respect to thevoltage. The measured phase is then compared to lower and upper phasethresholds to determine whether there is a short circuit or an opencircuit. The lower phase threshold may be from about 0.25 to about −1.6,in embodiments from about 0.1 to about −1.5. The upper phase thresholdmay be from about 1.1 to about 1.6, in embodiments, from about 1.25 toabout 1.5. If the measured phase rises above the upper threshold, anopen circuit is detected. If the measured phase falls below the lowerthreshold, a short circuit is detected.

If, in step 304, a short circuit is detected, e.g., the measuredimpedance falls below the low impedance threshold and/or the measuredphase rises above the upper phase threshold, or if an open circuit isdetected, e.g., the measured impedance rises above the high impedancethreshold and/or the measured phase falls below the lower phasethreshold, the control system or controller issues a regrasp alarm instep 360, and the tissue sealing procedure is exited in step 362. Theprocess is optionally reinitiated at step 300 (as shown by the dashedarrow connecting step 362 to step 300). Otherwise, if no fault isdetected in step 304 (i.e., no short and no open circuit is detected),the process starts the cook stage in step 306.

The control system then initiates application of RF energy (W_T) bydelivering current to the tissue so that the power calculated by thecontrol system tracks a non-linear polynomial power curve over time, instep 310. Once initiated, the control system increases the current sothat the applied power tracks the polynomial power curve, in step 312,until one of two events occurs: (i) the maximum allowable value of apredetermined peak of the polynomial power curve is reached, in step324; or (ii) the measured impendence value of the tissue exceeds sum ofthe lowest impedance value (Z_Low) and predetermined impedance risevalue (Z_Rise), in step 322.

During the cook stage the control system continuously measures thetissue impedance (e.g., Z_T) in step 314. The control system alsomeasures and stores the initial measured tissue impedance (e.g.,EndZ_Offset). During the polynomial power curve portion of the cookstage, the stored lowest impedance value (Z_Low) is updated anytime thecontrol system determines that a measured tissue impedance (Z_T) islower than the previous measured lowest impedance value (Z_Low). In step318, the control system compares the current measured tissue impedance(Z_T) with the stored lowest impedance value (Z_Low). If it isdetermined that the current measured tissue impedance (Z_T) is less thanthe previous measured lowest impedance value (Z_Low), this new value ofthe lowest impedance value (Z_Low) is stored in step 320, and thecontrol system continues to control the power of the RF energy so thatit tracks the polynomial power curve in step 312. If it is determinedthat the currently measured tissue impedance (Z_T) is greater than thepreviously measured lowest impedance value (Z_Low), the control systemproceeds to step 322. In other words, during steps 314, 318 and 320, thecontrol system waits for the tissue impedance to drop.

In steps 312, 314, 318, and 320, the control system controls the powerof the RF energy to track the polynomial power curve (examples of whichare shown in FIG. 4). When the currently measured tissue impedance (Z_T)is no longer less than the previously measured lowest impedance value(Z_Low) in step 318, the control system determines whether a tissuereaction has occurred, the control system uses two settings: (1) thelowest measured tissue impedance (Z_Low), and (2) the predeterminedimpedance rise value (Z_Rise), for the determination of a tissuereaction. The term “tissue reaction” is a point at which intracellularand/or extra-cellular fluid begins to boil and/or vaporize, resulting inan increase in tissue impedance. If the control system determines thatthe application of RF energy (W_T) has reached the maximum allowablepower value, the control system maintains the output power at themaximum allowable power value until the tissue “reacts.” This tissuereaction determination is based on whether the measured impedance (Z_T)is greater than a predetermined impedance threshold, such as the sum ofthe lowest impedance value (Z_Low) and the impedance rise value (Z_Rise)(i.e., the control system determines whether Z(t)>Z_Low+Z_Rise) (i.e.,the control system determines whether Z(t)>Z_Low+Z_Rise) in step 322

At step 322, if it is determined that the measured impedance (Z_T) isgreater than a predetermined impedance threshold, such as the sum of thelowest impedance value (Z_Low) and the impedance rise value (Z_Rise),the control system determines that a tissue reaction has occurred andexits the cook stage (proceeding to step 340) and initiates a coolingstage or phase, in step 342. The cooling stage may be an adaptive or anon-adaptive cooling stage, in which low power RF energy, e.g., RFenergy having a power less than the peak power, is applied to tissue.The adaptive cooling stage may control power of the RF energy based onone or more characteristics of tissue being treated, such as in a dZ/dtcontrol system, which varies the power of the RF energy so that changesin the measured impedance of the tissue tracks a desired changes inimpedance over time. The non-adaptive cooling stage may apply power thatis not based on one or more characteristics of tissue being treated,such as applying a constant low power or constantly decreasing power. Ifit is determined that a tissue reaction has not occurred, the controlsystem determines whether the application of RF energy (W_T) has reachedthe peak of the polynomial power curve (W_Peak), in step 324.

In step 324, if the application of RF energy (W_T) has reached the peakof the polynomial power curve (W_Peak) (as indicated, for example, byreference numbers 408 and 410 of FIG. 4, for various jaw groups), thecontrol system transitions the application of RF energy (W_T) to track alinear power curve, in step 326. If the application of RF energy (W_T)has not reached the peak of the polynomial power curve (W_Peak), in step324, the control system continues tracking the application of RF energy(W_T) to the polynomial power curve, in step 312.

Following the application of RF energy (W_T) to track the polynomialpower curve, in FIG. 3A, In step 326, the control system causes theapplication of RF energy (W_T) to track a linear power curve until oneof two events occurs: (i) a maximum allowable power value is reached; or(ii) the “tissue reacts.” In the event that the tissue reacts prior toreaching the maximum allowable power value, the energy required toinitiate a tissue “reaction” has been attained and the control systemexits the cook stage, in step 340, and transitions to a cooling stage,in step 342.

In step 328, the control system continuously measures the tissueimpedance (Z_T) in step 314. During the polynomial power curve portionof the cook stage, the stored lowest impedance value (Z_Low) is updatedanytime the control system determines that a measured tissue impedance(Z_T) is lower than the previous measured lowest impedance value(Z_Low). In step 332, the control system compares current measuredtissue impedance (Z_T) with the stored lowest impedance value (Z_Low).If it is determined that the current measured tissue impedance (Z_T) isless than the previous measured lowest impedance value (Z_Low) this newvalue of the lowest impedance value (Z_Low) is stored, in step 334, andthe applied power continues to track the linear power curve, in step326. If it is determined that the current measured tissue impedance(Z_T) is greater than the previous measured lowest impedance value(Z_Low), the control system proceeds to step 336.

In steps 326, 328, 332, and 334, the control system controls theapplication of RF energy (W_T) to track the linear power curve (examplesof which are shown in FIG. 4). Once the current measured tissueimpedance (Z_T) is greater than the previous measured lowest impedancevalue (Z_Low), in step 332, the control system then determines whether atissue reaction has occurred, the control system uses two settings: (1)the lowest measured tissue impedance (Z_Low) and (2) the predeterminedimpedance rise value (Z_Rise), for the determination of a tissuereaction. This tissue reaction determination is based on whether themeasured impedance (Z_T) is greater than a predetermined impedancethreshold, such as the sum of the lowest impedance value (Z_Low) and theimpedance rise value (Z_Rise) (i.e., the control system determineswhether Z(t)>Z_Low+Z_Rise), in step 336.

In step 322, if it is determined that the measured impedance (Z_T) isgreater than a predetermined impedance threshold, such as the sum of thelowest impedance value (Z_Low) and the impedance rise value (Z_Rise),the control system exits the cook stage, in step 340, and initiates anadaptive cooling stage, in step 342. In step 338, if it is determinedthat the measured impedance (Z_T) is not greater than a predeterminedimpedance threshold, such as the sum of the lowest impedance value(Z_Low) and the impedance rise value (Z_Rise), the control systemdetermines whether maximum cook stage timer has elapsed. If the maximumcook stage timer has not elapsed, in step 338, the application of RFenergy (W_T) continues to track the linear power curve in step 326.

In step 338, if the maximum cook stage timer has elapsed before themeasured impedance (Z_T) rises above the sum of the lowest impedancevalue (Z_Low) and the impedance rise value (Z_Rise), the control systemor controller issues a regrasp alarm in step 364, and the tissue sealingprocedure is exited in step 366. The process is optionally reinitiatedat step 300 (as shown by the dashed arrow connecting step 366 to step300).

After exiting the cook stage, in step 340, the control systemtransitions to a cooling stage, in step 342. One such example of acooling stage is an impedance matching trajectory based on the actualimpedance and the desired rate of change of impedance. The controlsystem calculates a target impedance value at each time step based on adesired rate of change of impedance (dZ/dt). The desired rate of changeof impedance may be stored as a setting in a configuration file and maybe loaded. The desired rise of impedance may be selected manually by auser or automatically based on the tissue type and/or instrumentcharacteristics. The control system then adjusts the power output fromthe output stage so that the measured tissue impedance tracks theimpedance trajectory. The impedance trajectory includes an initialimpedance value and extends over a period during which the tissuereaction is considered real and stable. In embodiments, the impedancetrajectory may be in the form of a non-linear and/or quasi-linear curve.

The target impedance trajectory includes a plurality of a targetimpedance values at each time step. The control system adjusts the powerlevel output from the output stage of the generator so that the measuredtissue impedance tracks the target impedance trajectory. The controlsystem determines whether tissue fusion is complete by determiningwhether the measured impedance rises above a predetermined thresholdimpedance and stays above the predetermined threshold impedance for apredetermined period. The predetermined threshold impedance may bedefined as an impedance level above an initial impedance value(EndZ_Offset). Using the predetermined threshold impedance minimizes thelikelihood of terminating electrosurgical energy early when the tissueis not properly or completely sealed. Further details regarding tissuesealing processes, including the cook stage, are found in commonly ownedU.S. patent application Ser. No. 13/483,815, entitled “System and Methodfor Tissue Sealing,” the entire contents of which are incorporated byreference herein.

FIG. 4 is a graph illustrating power control curves in two phases of thecook stage for two handsets of different sizes. Phase I includes anonlinear power curve and Phase II includes a linear power curve. BothGroups 1 and 2 are shown in FIG. 4. In the embodiment shown in FIG. 4,the nonlinear power curve is a polynomial power curve. As shown in FIG.4, the power control curves transition from the polynomial power curveto the linear power curve (e.g., a power ramp) at predetermined peakpowers 408 and 410. Although the graph illustrating power control curvesbegins each curve at minimum start power 402 which is shown at 10 W, itis contemplated that minimum start power 402 may be lower depending ontissue and generator requirements. In embodiments, the polynomial powercurve is customized for each handset to accommodate various jaw designs.For example, a higher power polynomial power curve may be used withlarger jaw designs while lower power polynomial power curves may be usedwith smaller jaw designs.

During the cook stage it is contemplated that, as shown in FIG. 4, themaximum cook stage time (step 338 of FIG. 3B) is 4 seconds. Once theapplication of RF energy (W_T) has continued for 4 seconds the controlsystem transitions from the cook stage, either issuing a regrasp alarm(step 364 of FIG. 3B) or determining that a tissue reaction has occurred(step 322 of FIG. 3A or step 336 of FIG. 3B). Additionally, the minimumamount of time for the application of RF energy (W_T) to continue in thecook stage is 0.5 seconds. If it is determined that a tissue reactionhas occurred prior to the 0.5 seconds, the control system will alsoissue a regrasp alarm and optionally restart the procedure.

As shown in FIG. 4, Group 1 is an example used for handsets with largerjaws and Group 2 is an example used for handsets with smaller jaws. Peakof the polynomial power curve (W_Peak) 408 for Group 2 for the smallerjaw handsets reaches its peak prior to peak of the polynomial powercurve (W_Peak) 410 for Group 2 for larger jaw handsets. The polynomialpower curve can be configured to vary how aggressively the power isincreased as shown in FIG. 4 to meet the specific devices sealing needs.

The power levels, shape, and length of the polynomial power curve may bedetermined based on parameters of the tissue or handset, such as tissuetype, tissue hydration values, initial tissue impedance, volume oftissue between jaw members 110 and 120, and/or surface area of jawmembers 110 and 120. For example, these parameters may be used todetermine the predetermined peak power and the length of the polynomialpower curve. The parameters of the polynomial power curve, such as thepredetermined peak power and the length or duration of the polynomialpower curve, may be adjusted based on desired cook stage results. Thenonlinear power curve may include second-order, third-order, or greaterorders of polynomial powers based on parameters of the tissue or handsetdescribed above. One example of a third-order polynomial is defined bythe formula:

y=ax ³ +bx ² +cx+d.  (1)

Through the selection of the coefficients a, b, c, and d, the curve of agraph plotting the polynomial power curve can be configured to match adesired slope, peak value, y-intercept, and curvature.

Although the example of the polynomial power curve portion of FIG. 4 isillustrated as a third order polynomial, greater orders may be used andvia the selection of the coefficients used in the polynomial formula thecurve may be configured, such that undesirable powers of the polynomialformula are removed through the use of a zero coefficient.

The curve of the polynomial power curve allows the cook stage to applypower to tissue slowly at first. The coefficients used in the polynomialpower curve allow for customization of the power curve, such as theduration of time at which the power curve reaches the peak or the rateat which the slope of the polynomial power curve increases and/ordecreases. One such example of a curve configuration is shown in FIG. 4,this curve has been customized to include an increasing power throughoutthe polynomial power curve portion via a slow start followed by anincreased ramping and a slow ending. This slow application of powerlimits the risk of incidental impedance spikes due to over powering thetissue which may cause the control system to inadvertently leave thecook stage state prematurely before a sufficient amount of power hasbeen applied to the tissue.

When the power of the electrosurgical energy output from the outputstage reaches the predetermined peak powers 408 and 410, the controlsystem controls the power of the electrosurgical energy according to thelinear power curve. For example, the control system linearly ramps thepower according to a desired slope as shown in FIG. 3B.

Referring back to FIG. 2, memory 6 includes any non-transitorycomputer-readable storage media for storing data and/or software that isexecutable by microprocessor 5 and which controls the operation ofcontroller 4. In embodiments, memory 6 may include one or moresolid-state storage devices such as flash memory chips. Alternatively orin addition to the one or more solid-state storage devices, memory 6 mayinclude one or more mass storage devices connected to the microprocessor5 through a mass storage controller (not shown) and a communications bus(not shown).

Although the description of computer-readable media refers to asolid-state storage, it should be appreciated by those skilled in theart that computer-readable storage media can be any available media thatcan be accessed by the microprocessor 5. That is, computer readablestorage media includes non-transitory, volatile and non-volatile,removable and non-removable media implemented in any method ortechnology for storage of information such as computer-readableinstructions, data structures, program modules or other data. Forexample, computer-readable storage media includes RAM, ROM, EPROM,EEPROM, flash memory or other solid state memory technology, CD-ROM,DVD, Blu-Ray or other optical storage, magnetic cassettes, magnetictape, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to store the desired information.

The detailed embodiments described in the present application are merelyexamples of the disclosure, which may be embodied in various forms.Therefore, specific structural and functional details disclosed hereinare not to be interpreted as limiting, but merely as a basis for theclaims and as a representative basis for allowing one skilled in the artto variously employ the present disclosure in virtually anyappropriately detailed structure.

What is claimed is:
 1. An electrosurgical generator, comprising: a radiofrequency (RF) output stage configured to supply power to tissue;sensing circuitry configured to measure impedance of tissue; and acontroller configured to: control the power supplied from the RF outputstage to track a nonlinear power curve until the power supplied from theRF output stage has reached a predetermined peak power of the nonlinearpower curve; determine whether a tissue reaction has occurred based onimpedance measured by the sensing circuitry; and control the powersupplied from the RF output stage during a cooling phase if thecontroller determines that a tissue reaction has occurred.
 2. Theelectrosurgical generator according to claim 1, wherein the controlleris further configured to control the power supplied from the RF outputstage to track a linear power curve if the controller determines thatthe power supplied from the RF output stage has reached thepredetermined peak power of the nonlinear power curve.
 3. Theelectrosurgical generator according to claim 2, wherein the controllerincludes a memory storing a look-up table including a plurality ofnonlinear power curves, a respective plurality of linear power curves,and a respective plurality of sizes of electrodes of electrosurgicalinstruments usable with the electrosurgical generator, and wherein thecontroller is configured to receive an electrode size and select anonlinear power curve from the plurality of nonlinear power curves and arespective linear power curve from the plurality of linear power curvesbased on the received electrode size.
 4. The electrosurgical generatoraccording to claim 1, wherein the controller is further configured toadjust a parameter of the linear power curve based on a surface area ofan electrode of an electrosurgical instrument usable with theelectrosurgical generator, and wherein the parameter of the linear powercurve is selected from the group consisting of slope and duration. 5.The electrosurgical generator according to claim 4, wherein the durationof the linear power curve is zero.
 6. The electrosurgical generatoraccording to claim 4, wherein the linear power curve has a shorterduration and a larger slope for a larger surface area of the electrode.7. The electrosurgical generator according to claim 4, wherein thelinear power curve has a longer duration and a smaller slope for asmaller surface area of the electrode.
 8. The electrosurgical generatoraccording to claim 1, wherein the controller is further configured toadjust a parameter of the nonlinear power curve based on a surface areaof an electrode of an electrosurgical instrument usable with theelectrosurgical generator.
 9. The electrosurgical generator according toclaim 8, wherein the parameter of the nonlinear power curve is selectedfrom the group consisting of starting power, duration, shape, slopes,the predetermined peak power, and combinations thereof.
 10. Theelectrosurgical generator according to claim 8, wherein the nonlinearpower curve has a longer duration and a smaller predetermined peak powerfor a larger surface area of the electrode.
 11. The electrosurgicalgenerator according to claim 8, wherein the nonlinear power curve has ashorter duration and a larger predetermined peak power for a smallersurface area of the electrode.
 12. The electrosurgical generatoraccording to claim 8, wherein the electrosurgical instrument includes aRadio Frequency Identification tag storing the parameter of thenonlinear power curve.
 13. The electrosurgical generator according toclaim 1, wherein the nonlinear power curve is a third-order or cubicpolynomial defined by a plurality of coefficients.
 14. Theelectrosurgical generator according to claim 1, wherein the controllerincludes a memory storing a look-up table including a plurality ofnonlinear power curves and a respective plurality of sizes of electrodesof an electrosurgical instrument usable with the electrosurgicalgenerator, and wherein the controller is configured to receive anelectrode size and select a nonlinear power curve from the plurality ofnonlinear power curves based on the received electrode size.
 15. Theelectrosurgical generator according to claim 1, wherein the controlleris further configured to: determine a minimum impedance based on themeasured impedance; and determine whether a tissue reaction has occurredbased on the minimum impedance and a predetermined rise in impedance oftissue being treated.
 16. The electrosurgical generator according toclaim 1, wherein the controller is further configured to determinewhether a tissue reaction has occurred within a first predeterminedperiod, stop the power supplied from the RF output stage, and issue are-grasp message, if the controller determines that a tissue reactionhas occurred within the first predetermined period.
 17. Theelectrosurgical generator according to claim 1, wherein the controlleris further configured to determine whether a tissue reaction hasoccurred within a second predetermined period, and control the powersupplied from the RF output stage to restart tracking of the nonlinearpower curve if the controller determines that a tissue reaction has notoccurred within the second predetermined period.
 18. A method forperforming an electrosurgical procedure comprising: supplying power totissue from a radio frequency (RF) output stage; measuring impedance oftissue; controlling the power supplied to track a nonlinear power curveuntil the power supplied of has reached a predetermined peak power ofthe nonlinear power curve; determining whether a tissue reaction hasoccurred based on measured impedance of tissue; and controlling thepower supplied according to a cooling phase if it is determined that atissue reaction has occurred.
 19. The method according to claim 18,further comprising controlling the power supplied from the RF outputstage to track a linear power curve if it is determined that the powersupplied from the RF output stage has reached the predetermined peakpower of the nonlinear power curve.
 20. An electrosurgical system,comprising: an electrosurgical generator including: a radio frequency(RF) output stage configured to supply power to tissue; sensingcircuitry configured to measure impedance of tissue; and a controllerconfigured to: control the power supplied from the RF output stage totrack a nonlinear power curve; determine whether the power supplied fromthe RF output stage has reached a predetermined peak power of thenonlinear power curve; control the power supplied from the RF outputstage to track a linear power curve if the controller determines thatthe power supplied from the RF output stage has reached thepredetermined peak power of the nonlinear power curve; determine whethera tissue reaction has occurred based on impedance measured by thesensing circuitry; and control the power supplied from the RF outputstage according to a cooling phase if the controller determines that atissue reaction has occurred; and an electrosurgical instrument coupledto the electrosurgical generator, the electrosurgical instrumentincluding at least one electrode.